Removing thrust::optional on device_uvector until it can be sorted out

[hyperbolic2346]

This does some cleanup on eclipsed variables and, more importantly, fixes the compilation error described in #7629 by removing the usage of thrust::optional with a device_uvector.

This will be cleaned up once device_uvector is addressed, but until then this will unblock the build.

[codecov]

Codecov Report

Merging #7630 (31eb436) into branch-0.19 (7871e7a) will increase coverage by 0.52%.
The diff coverage is 93.84%.

❗ Current head 31eb436 differs from pull request most recent head 8baa2b7. Consider uploading reports for the commit 8baa2b7 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.19    #7630      +/-   ##
===============================================
+ Coverage        81.86%   82.39%   +0.52%     
===============================================
  Files              101      101              
  Lines            16884    17352     +468     
===============================================
+ Hits             13822    14297     +475     
+ Misses            3062     3055       -7     

Impacted Files	Coverage Δ
python/cudf/cudf/core/index.py	93.34% <ø> (+0.48%)	⬆️
python/cudf/cudf/core/column/numerical.py	94.83% <87.50%> (-0.20%)	⬇️
python/cudf/cudf/core/frame.py	89.09% <89.47%> (+0.08%)	⬆️
python/cudf/cudf/core/column/column.py	87.86% <90.00%> (+0.10%)	⬆️
python/cudf/cudf/core/column/decimal.py	92.75% <90.32%> (-2.12%)	⬇️
python/cudf/cudf/core/dataframe.py	90.58% <95.65%> (+0.11%)	⬆️
python/cudf/cudf/core/series.py	91.57% <95.83%> (+0.79%)	⬆️
python/cudf/cudf/core/column/categorical.py	91.97% <100.00%> (+0.58%)	⬆️
python/cudf/cudf/core/column/datetime.py	89.63% <100.00%> (+0.54%)	⬆️
python/cudf/cudf/core/column/string.py	86.79% <100.00%> (+0.30%)	⬆️
... and 59 more

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[robertmaynard]

I believe that with rapidsai/rmm#734 now merged we don't need this

[hyperbolic2346]

I agree, thanks for getting that in! I would still like some of the cleanup here, but I will use a different PR for that without the removed optional.